1. Field of the Invention
The present invention relates to optically and/or electronically active and/or passive semiconductor devices having a nitride-based hetero-structure mainly composed of InN or InN-based compounds (e.g., semiconductor laser diodes/light emitting diodes which have an excellent temperature characteristic to be used as a light source in optical communication, ultra-high speed optical control devices/optical modulators capable of working in a femto second range, resonant tunnel diodes, ultra-high speed and ultra-high power and ultra-high power saving electronic devices, etc.), and to a method of manufacturing the same. The term “a compound mainly composed of InN” used herein refers to an InN-based compound which contains InN at 50% or more. The term “nitrogen (N) polarity surface or a surface equivalent to that surface” used herein refers to a polarized surface such as (001) or (101) plane having N polarity, and surfaces tilted 10 degrees or less from that surface.
2. Description of the Related Art
Nitride semiconductors mainly composed of gallium nitride (GaN) can be used as a material of UV and visible light emitting elements and ultra-high speed electronic devices. According to recent studies, it was found that indium nitride (InN) has an energy band-gap of about 0.7 eV or a value considerably lower than the hitherto reported one. From this it was suggested that InN-containing nitride semiconductors could cover a considerably wide energy band-gap range whose lower limit reaches as low as 0.7 eV. Further, the difference in the energy band-gap between InN and GaN becomes very large, resulting in such a large conduction band offset as about 2 eV similar to the case in that between GaN and AlN. From this it is expected that nitrides semiconductor based hetero-structure devices mainly composed of InN or an InN-based compound will be stable over a considerably wide temperature range, and thus they will be profitably used as a material of basic optical/electronic devices to support ultra-high speed and ultra-broad bandwidth optical communication expected to be introduced in the near future for transmitting a huge amount of visual information, for example, as a light source to support such optical communication, or as a power amplifier to amplify signals at a relay station of a network involved in such optical communication.
Aluminum nitride (AlN), gallium nitride (GaN) and indium nitride (InN)-based nitride semiconductors have a hexagonal crystal structure. As for many hetero-structure devices obtained by depositing such semiconductors on a sapphire or silicon carbide (SiC) substrate, the c-axis of the crystal is essentially normal to the surface of the substrate. When epitaxy is used for growing AlN or GaN crystal, it is preferably carried out in such a manner as to allow the crystal to grow along the c-axis to exhibit cation elements such as Al or Ga on the surface of growing crystal (+c polarity). However, what effect the polarity of crystal mainly composed of InN or an InN-based compounds has on the epitaxy itself and the quality of the epitaxial film still remains unclear. Generally, however, it has been believed that nitride semiconductor materials, similarly to those composed of GaN, having +c polarity are preferable. For details in this point, see, for example, the following non-patent documents: Y. Sato et al., “Polarity of high quality indium nitride grown by RF molecular beam epitaxy,” Phys. Stat. Sol., (b) 228, No. 1, (2001), pp. 13-16, and A. Yoshikawa, et al., “In situ investigation for polarity-controlled epitaxy processes of GaN and AlN in MBE and MOVPE growth,” Optical Materials 23, (2003), pp. 7-14.
The crystal growth (hetero-epitaxy) itself of a compound semiconductor material mainly composed of InN or an InN-based compound is extremely difficult because the equilibrium vapor pressure of InN is very high. For the resulting epitaxial film to be suitably used as a material of optical devices for optical communication or of high-speed and high-power and high power saving electronic devices, it must have an ultra-thin thickness and ultra-abrupt hetero-structural interfaces. However, it is difficult at present to obtain epitaxial films of InN or InN-based compounds which are atomically flat as is achieved by a so-called step flow growth process for other semiconductor materials. Moreover, the crystal quality of an epitaxial film obtained by a conventional process is poor and not enough for device applications. Thus, the conventional epitaxy process must be significantly improved so as to produce an epitaxy film from InN or an InN based compound exhibiting a satisfactory crystal quality.